1. Technical Field
The present disclosure relates to a driving circuit for a microelectromechanical gyroscope and to a related microelectromechanical gyroscope.
2. Description of the Related Art
As is known, micromachining techniques enable manufacturing of microelectromechanical structures or systems (MEMS) within layers of semiconductor material, which have been deposited (for example, a layer of polycrystalline silicon) or grown (for example, an epitaxial layer) on top of sacrificial layers, which are removed via chemical etching.
Inertial sensors, such as accelerometers and gyroscopes, built using said technology are enjoying an increasing success, for example, in the automotive field, in inertial navigation, or in the sector of portable devices.
In particular, integrated gyroscopes made of semiconductor material using MEMS technology are known. These gyroscopes operate based on the theorem of relative accelerations, exploiting Coriolis acceleration. When an angular velocity is applied to a mobile mass that is driven with a linear velocity, the mobile mass “feels” an apparent force, known as Coriolis force, which determines a displacement thereof in a direction perpendicular to the direction of the linear velocity and to the axis about which the angular velocity is applied. The mobile mass is supported via springs that enable a relative displacement thereof in the direction of the apparent force. According to Hooke's law, the displacement is proportional to the apparent force, in such a way that, from the displacement of the mobile mass, it is possible to derive the Coriolis force and the value of the angular velocity that has generated it.
The displacement of the mobile mass may, for example, be detected in a capacitive manner by determining, in a resonance condition, the variations of capacitance (or, likewise of the quantity of charge) caused by the movement of mobile electrodes, fixed with respect to the mobile mass (or constituted by parts of the same mobile mass) and coupled to fixed electrodes.
In greater detail, MEMS gyroscopes have a rather complex electromechanical structure, which comprises two masses that are mobile with respect to a same fixed body (usually defined as “stator”) and are coupled together so as to have one relative degree of freedom.
One of the mobile masses is dedicated to driving (and for this reason is commonly defined as “driving mass”) and is kept in oscillation at its resonance frequency by means of a suitable electronic driving circuit (or forcing circuit). The other mobile mass (usually known as “sensing mass”) is drawn along in the oscillatory motion by the driving mass and, in case of rotation of the microstructure with respect to a preset axis at a given angular velocity, is subject to a Coriolis force proportional to the same angular velocity. The sensing mass thus operates as an accelerometer that enables detection of Coriolis acceleration.
As regards implementation of the electronic driving circuit, a first known solution envisages supplying, in open loop, of periodic stresses at the resonance frequency of the MEMS structure. The solution is simple, but also far from effective, because the resonance frequency is not known precisely on account of the inherent dispersions in the processes of micromachining of semiconductor materials. Moreover, the resonance frequency of each individual device may vary over time, for example, on account of temperature gradients or, more simply, owing to ageing.
Feedback driving stages have thus been proposed, in which a feedback loop is used for controlling the driving signal so as to maintain the resonance condition.
In particular, in order to enable driving and provide an electromechanical oscillator in which the sensor mechanical structure performs a role of selective frequency amplifier with a second-order transfer function of a low-pass type and high merit factor (the so-called “Q factor”, equal to at least 20, but also of the order of 100 or 1000), the driving mass is coupled to two differential capacitive structures: a set of driving electrodes and a set of driving-sensing electrodes.
The driving electrodes have the purpose of enabling sustaining of self-oscillation of the mobile driving mass in the driving direction, by means of electrostatic forces generated by the spectral component of the noise at the mechanical resonance frequency of the driving mass. In particular, a driving signal is applied to the driving electrodes, for example, a voltage signal of the square-wave type, with suitable amplitude and frequency such as to cause resonance oscillation.
The driving-sensing electrodes have the purpose of enabling detection, through the transduced charge, of the translation or rotation movement of the driving mass in the driving direction in such a way as to enable a feedback control of the driving signal. In particular, movement detection is performed continuously, by processing analog signals, generally voltage signals.
Although effective for controlling resonance oscillation of the driving mass, a solution of this type is not, however, optimized from the standpoint of area occupation and costs on account of the additional presence of a set of electrodes dedicated to detection of the driving movement, which do not contribute to the system operation and to the detection of angular velocities. In particular, this solution may prove for this reason not applicable in systems where size reduction represents a fundamental design criterion.